1. Field of the Invention
The invention relates to a signal coupler for telephone lines containing both plain old telephone service (POTS) and digital signals where low voltage filtering is used in the POTS channel.
2. Description of the Related Art
Modern data networks commonly use complex digital signal processing (DSP) devices called modems to transport data over communication channels. Data is typically transported via an analog transmission signal which is representative of a synchronous, constant rate bit stream. This form of communication channel is suitable for the transmission of real-time information such as voice or video.
Often it is desirable to transmit both Plain Old Telephone Service (POTS) and digital data, either by Asymmetric Digital Subscriber Line (ADSL) or some other method, over the same line. The POTS frequency spectrum ranges from 300 to 3400 Hz. The ADSL frequency spectrum ranges from 24 kHz to 1100 kHz.
As shown in FIG. 1, the data and POTS signals are transmitted over standard telephone lines between a central office and a subscriber""s home. The subscriber may have several modems and a POTS service. The subscriber is typically connected to a central office by twisted copper wire pair. At the central office, a signal coupler is used to filter, split and digitize signals coming into the central office from a subscriber. The digital signals are processed through switching networks and then sent through another signal coupler to another subscriber. Alternatively, the digital signal may be transmitted to another central office before being sent through another signal coupler to another subscriber. The signal coupler converts the digital signals from the switching circuits into analog signals for transmission to the subscriber as well as converting the analog input from the subscriber into digital signals which are sent to the switching circuits.
The transmission lines between the central office and the subscriber may be twisted copper pairs, as shown in FIG. 1. Other possibilities for transmission lines include fiber optics. In any case, the equipment at the central office and at the subscriber must be protected against power cross, lightning strikes, or other high voltage events and current surges on the telephone line. The main voltage protection is accomplished outside of the central office. However, secondary voltage protection is usually included in the signal coupler. In FIG. 2, the signal from the subscriber appears on the TIP and RING lines and the secondary voltage protection is shown as the circuit protection 101.
After the voltage and current protection is accomplished, the signal is split into data and voice lines. The data is sent at frequencies in the 20-30 kilohertz range and up while POTS voice information is nominally below 3000 Hz. The splitting, then, is normally done by using a high-pass filter for the data lines and a series of low-pass filters for the POTS lines. The series of filters is further required to remove the noise from the incoming telephone cable. FIG. 2 shows the typical filtering circuit for the POTS. In FIG. 2, the voice filtering is accomplished by a multistage filtering circuit. The typical low pass filter used in the multistage filtering circuit has two to four stages of filtering.
The POTS signal from the subscriber, after passing through the protection circuit 101, is filtered by the multistage low pass filter 102. The multiple stages allow for filtering of multiple orders (i.e., one stage provides two orders of filter, thus multiple stages provides multiples of second order filtering). The components of each stage include an inductor pair and a capacitor. Stage 1 in FIG. 2, for example, has inductor L1 connected in series with the TIP line after protection circuit 101 and inductor L1xe2x80x2 connected in series with the RING line after the protection circuit. Capacitor C1 is connected across the TIP and RING lines after the inductor pair L1 and L1xe2x80x2. The remaining stages have similar components. In addition, the low pass filter 102 includes resistors R1 and R2 connected in series with the TIP and RING lines before they are connected to Stage 1 and resistors R3 and R4 connected in series with the IP and RING lines after they exist stage N. In addition, the low pass filtering circuit may include an additional inductor pair acting as a common mode choke which rejects signals common to both input lines. The common mode choke is connected before stage 1 and is connected similar to inductor pair L1 and L1xe2x80x2.
Each component of the filtering circuit (i.e., the resistors, inductors, and capacitors) must be capable of withstanding the high battery voltage (48 V) and high DC currents (typically 25 mA) used in the subscriber loop. This necessitates that each of the components in the filtering circuit be discrete components which require a large amount of space on the circuit board. This space requirement restricts the number of lines that can be placed on a given circuit board. It is desirable, then, to reduce the amount of filtering which must be accomplished in the high voltage mode.
Accordingly, the present invention provides a signal coupler circuit which provides the circuit protection and the filtering in a way that allows conservation of space on the circuit board at the central office. This is done by filtering the voice lines, at least in part, in a portion of the circuit where the high battery voltage is no longer present.
An analog signal capable of containing both POTS and data signals is present between the TIP and RING lines. The TIP and RING lines are the components of a standard twisted pair configuration of telephone service. The invention, however, is not restricted to twisted pairs and is useful for any transmission method of telephone service.
The signal between TIP and RING is first inputted to a voltage surge protection circuit. The surge protection circuit limits voltage spikes and current surges which could damage other components. The main voltage and current surge protection is connected to the transmission line outside of the central office so that the protection required on the circuit board inside the central office is secondary circuit protection.
The output signal from the protection circuit is inputted to a low-pass filter. The low-pass filter occupies a minimal amount of space while being capable of accommodating the high voltage telephone lines. In the preferred embodiment, only one stage of filtering is used. More stages of filtering could be used at this point but, because of the high battery voltage, these stages utilize a great deal of circuit board space.
The output signal from the low-pass filter is then sent to a standard subscriber loop interface circuit (SLIC) device. The SLIC is a standard chip which splits the two incoming lines into four lines, a pair of receive lines and a pair of transmit lines.
The transmit lines are finally filtered after the SLIC chip to remove the remainder of the noise. When the filtering is done after the signal is passed through the SLIC component, the high battery voltage is no longer present. The filtering components, then, can exist on a single chip and will take up much less space on the circuit board. Alternatively, some of the filtering may be accomplished digitally after the signal has been digitized by the CODEC.
The present invention provides a way of dispensing with the multistage high-voltage filtering which typically occurs before the SLIC in favor of low voltage, and therefore smaller and more compact, filtering on the transmit lines after the SLIC. The resulting savings in space will allow more signal couplers to exist on a given circuit board.